docs: def download_sifts_xml(pdb_id, outdir='', force_rerun=False):
      """Download the SIFTS file for a PDB ID.
  
      Args:
          pdb_id (str): PDB ID
          outdir (str): Output directory, current working directory if not specified.
          force_rerun (bool): If the file should be downloaded again even if it exists
  
      Returns:
          str: Path to downloaded file
  
      """
      baseURL = 'ftp://ftp.ebi.ac.uk/pub/databases/msd/sifts/xml/'
      filename = '{}.xml.gz'.format(pdb_id.lower())
  
      outfile = op.join(outdir, filename.split('.')[0] + '.sifts.xml')
  
      if ssbio.utils.force_rerun(flag=force_rerun, outfile=outfile):
          response = urlopen(baseURL + filename)
          with open(outfile, 'wb') as f:
              f.write(gzip.decompress(response.read()))
  
      return outfile